1. Field of the Invention
The invention relates to pipeline pig detectors and more particularly relates to an improved apparatus and method for standardizing a pig detector's trip point so any standardized detector will reproduceably and interchangeably signal when a pig reaches the same point in a pipeline.
2. Description of the Prior Art
A solid object such as a sphere or scraper, introduced into a pipeline and propelled by the fluid flowing therein, is commonly called a pig. It is sometimes necessary to detect the pig's position for various reasons. Typically, pig detectors employ a mechanical sensing means, such as a moveable trigger, to contact the pig. The trigger is mechanically coupled to a reporting means, such as a switch or valve, and is adapted to actuate this reporting means when the trigger is moved by the pig. An excellent example of such a pig detector is disclosed in U.S. Pat. No. 3,145,274, which is here incorporated by reference. Such detectors adequately perform the gross function of reporting a pig's approximate location in the pipeline.
One use of a pig detector is as a sensor in a device known as a meter prover. A meter prover is a precision flow meter used to calibrate commercial flow meters. They are generally accurate to 0.02%. Typically a pig is propelled by flowing fluid through a measured length of pipe having a known cross-section. The amount of fluid passing through the tube may be determined from the length of time required for the pig to travel from beginning to end of the measured section. In this device, the pig detector performs the critical role of sensing and reporting the pig's entry into and exit from the measured section. An excellent example of a meter prover is disclosed by U.S. Pat. No. 3,738,153, which is now incorporated by reference.
Calibration of a meter prover is difficult, expensive and time consuming. Once the system is calibrated, however, it yields highly accurate results. Virtually the only moving parts that wear out in a meter prover are the pig and the pig detectors. The pig is generally a sphere and may be replaced by any other sphere of congruent geometry made from similar material. Replacing the meter prover presents a more difficult problem.
When a pig detector is removed from the meter prover for repair or replacement, it is necessary to volumetrically re-calibrate the entire meter prover system. This is because the trigger of the new or repaired detector activates its associated switch or valve when the pig is in a slightly different position in the tube than did the previously mounted pig detector. In most uses these differences are so slight they may safely be ignored. Unfortunately, in precision meter provers these differences in activation points cause unacceptably large measurement errors and necessitate the aforementioned lengthy and costly re-calibration.